Life is full of risks. Getting the next meal or defending your territory can be hazardous. David McKenzie from the CNRS Montpellier, France, explains that animals gamble with fate for several reasons. Some animals have to place themselves at risk when foraging to satisfy a high metabolic rate, while others are simply bolder: and fish are no exception. McKenzie is also intrigued by air-breathing fish and was puzzled why less than 2% of fish have evolved the ability. ‘Air is a rich source of oxygen but breathing it is risky, so it made me wonder if it might be linked to personality’, says McKenzie, explaining that fish that breathe at the surface are vulnerable to predation. Knowing that Brazil is gripped by an invasion of air-breathing African sharptooth catfish (Clarias gariepinus), McKenzie decided to visit his collaborator Tadeu Rantin at the Federal University of São Carlos, Brazil, to find out whether it takes courage for the catfish to breathe air or whether they simply surface to satisfy a fast metabolism.
Collecting feral fish in São Paulo state and transporting them back to the lab, McKenzie, Rantin and Thiago Belão then developed a bimodal respirometer – with a space to allow individual catfish to breathe air – to measure the animal's air- and water-oxygen consumption simultaneously. After allowing the fish to rest for 24 h in the respirometer, McKenzie and Belão then recorded the amount of oxygen that they consumed from air and water to determine their metabolic rates. Next, they startled the fish by tapping on the respirometer and recorded how long it took the animals to pluck up the courage to return to the surface. ‘We assumed that how quickly they returned to air breathing was a measure of how bold they were’, says McKenzie.
Initially, they found that the fish in fully oxygenated water did not need to breathe oxygen from the air during the day, although they did so anyway. However, the fish in oxygen-depleted water were clearly struggling to meet their metabolic demands and resorted to air breathing often. And when the team plotted a graph of the fish's metabolic rate against the time that it took them to resume air breathing after a fright, they could see that the fish that had the highest metabolic rates breathed air the most. ‘There was an element of strong drive of individual metabolic rate to air breathe’, says McKenzie.
But McKenzie also wondered whether boldness could be another factor affecting how swiftly the fish resumed air breathing. Maybe some of the fish were more courageous than others and this was being obscured by the fish's strong urge to breathe air. Recalling colleagues suggesting a technique to help tease apart the impact of boldness from the metabolic drive to breathe air, McKenzie and Shaun Killen from the University of Glasgow scrutinised the data again and realised that the fish could be divided into two groups – one timid and the other bold – with the bold fish returning to the surface to breathe within 75 min of a fright, while the timid animals remained cowering at the bottom for at least 115 min. Also, the bold animals tended to air breathe more during the day and they chose to breathe air in well-oxygenated water, even though there was no need to place themselves at risk. ‘The bold individuals were apparently just ‘choosing’ to [put themselves at risk]’, says McKenzie. Adding that physiology and personality are not necessarily linked, he says, ‘Some animals take risks because they have to, others seem to take risks just because they want to’.